Light distribution and spectral composition within cultures of micro-algae: Quantitative modelling of the light field in photobioreactors

Abstract

[EN] Light, being the fundamental energy source to sustain life on Earth, is the external factor with the strongest impact on photosynthetic microorganisms. Moreover, when considering biotechnological applications such as the production of energy carriers and commodities in photobioreactors, light supply within the reactor volume is one of the main limiting factors for an efficient system. Thus, the prediction of light availability and its spectral distribution is of fundamental importance for the productivity of photo-biological processes. The light field model here presented is able to predict the intensity and spectral distribution of light throughout the reactor volume. The input data for the algorithm are chlorophyll-specific absorption and scattering spectra at different irradiance values for a given organism, the depth of the photobioreactor, the cell-density and also the intensity and emission spectrum of the light source. Although in the form exposed here the model is optimized for photosynthetic microorganism cultures inside flat-type photobioreactors, the theoretical framework is easily extensible to other geometries. Our calculation scheme has been applied to model the light field inside Synechocystis sp. PCC 6803 wild-type and Olive antenna mutant cultures at different cell-density concentrations exposed to LED lamps of different colours, delivering results with reasonable accuracy, despite the data uncertainties. To achieve this, Synechocystis experimental attenuation profiles for different light sources were estimated by means of the Beer- Lambert law, whereby the corresponding downward irradiance attenuation coefficients were obtained through inherent optical properties at any wavelength within the photosynthetically active radiation band. In summary, the model is a general tool to predict light availability inside photosynthetic microorganism cultures and to optimize light supply, in respect to both intensity and spectral distribution, in technological applications. This knowledge is crucial for industrialscale optimisation of light distribution within photobioreactors and a fundamental parameter for unravelling the nature of many photosynthetic processes.